SPACE-TIME VARIATIONS IN SOURCE PARAMETERS OF MINING-INDUCED SEISMIC EVENTS WITH M < 0

Abstract

Space-time variations in source parameters and fault-plane solutions are related to structural mapping data and local/regional stress field measurements in an attempt to characterize the behavior of a rock mass in the vicinity of an excavation in a mine. Source parameters (i.e., total seismic energy, P - and S -wave energy, seismic moment, static stress drop, and apparent stress) were analyzed for 85 events with moment magnitudes between −2.2 < M < −0.3 within a 90-m microseismic envelope ahead of an excavation. These data allowed three types of failure mechanisms to be postulated: a zone of flat-lying extensional failures next to the excavation; a zone of subvertical shear failures at the microseismic envelope boundary; and a transition zone between these two where both shear and shear-extensional failures occur. Two spatially distinct event clusters occurred at distance from the excavation for early and late stages of the excavation, and next to the excavation during intermediate stages. Worth noting was an apparent change in P -axis orientation with excavation, where the average P axes before and following ore extraction were compatible with the local maximum principal stress and the P axes during intermediate excavation stages were compatible with the regional maximum principal stress (or local intermediate principal stress). Distinct from these trends was the general increase in the values of each of the above source parameters over the duration of the excavation, suggesting that stress levels were increasing as a result of the excavation geometry and procedure. This was considered to be significant as rockbursts ( mN 1.8 to 2.7) occurred above the study area 6 months following the completion of the excavation.